1. Field of the invention
The present invention relates to an optical feedback photodetection apparatus for detecting a change in intensity of a light beam emitted from a light source, which change is caused due to a light beam returned from an object to be measured.
2. Related background art
When a light beam emitted from a laser resonator irradiates some substance, and part of the light beam is reflected by the substance and fed back to the laser resonator, a large change is caused in laser characteristics even if the relative feedback amount is very small. Such variations in characteristics caused due to the return light beam from the outside of the laser are particularly conspicuous for a semiconductor laser, resulting in large disadvantages including an increase in noise in various application fields.
On the other hand, there is a technique called a laser feedback photodetection method positively using the above-mentioned phenomenon, in which a light beam emitted from a semiconductor laser is irradiated on a predetermined object, and the intensity of the light beam reflected, scattered, or diffracted by the object is detected and measured.
Unlike a technique for directly detecting the intensity of the light beam reflected or scattered by the object to be measured, the laser feedback photodetection method requires none of an optical isolator for suppressing the return light beam to the semiconductor laser serving as a light source, a beam splitting means for guiding the light beam reflected or scattered by the object to the photodetector, and a pinhole plate for preventing noise from being generated in the photodetector. Therefore, the optical arrangement can be largely simplified.
For this reason, this method has been regarded as a promising optical disk pickup technique which requires a very simple and compact optical system and studied ("Optics Communications Handbook" edited by Hisayoshi Yanai, published by Asakura Shoten, 1984, pp. 610-611; and Y. Mitsuhashi, et at., Optics Communications. April 1976, Vol. 17, pp. 95-97).
In recent years, a study group in the Oxford University has reported that an excellent result can be obtained by applying the laser feedback photodetection method to a confocal laser scanning microscope (R. Juskaitis, et al., Optics Communications, 109 (1994) pp. 167-177; and R. Juskaitis, et al., Optics Letters. July 1993, Vol. 18, No. 14, pp. 1135-1137).